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Damper Throttling Operating Point Explorer

Run the Damper Throttling Operating Point Explorer MicroSim Fullscreen

About This MicroSim

The chart shows one illustrative fan curve and the system curve it works against. Damper throttling multiplies the system's resistance coefficient by a factor m, so the system curve becomes steeper each time the damper closes:

  • Fan curve: P = 6.0 − (Q / 10,000)² in. wg
  • System curve: P = 5.0 × m × (Q / 10,000)² in. wg
  • Operating point: Q = 10,000 × √(6.0 / (1 + 5.0m)) and P = 6.0 − 6.0 / (1 + 5.0m)

The damper closes in five fixed steps. Before each new system curve is drawn, the learner predicts whether airflow will be higher, lower or the same, and whether static pressure will be higher, lower or the same. Pressing Check locks the pair, draws the new system curve and its operating point, and keeps the previous system curve (dotted grey) and the previous operating point (open circle) on the chart for comparison. A table beside the chart lists every operating point revealed so far.

Damper setting Multiplier m Airflow (CFM) Static pressure (in. wg)
Wide open 1.0 10,000 5.00
Step 1 1.5 8,402 5.29
Step 2 2.0 7,385 5.45
Step 3 3.0 6,124 5.63
Step 4 4.0 5,345 5.71
Step 5 6.0 4,399 5.81

Each step has one scored attempt. A prediction is correct only when both directions are right (airflow lower, static pressure higher). A wrong pressure direction and a wrong airflow direction each get their own explanation. After Step 5 the summary reads "Predictions correct: n of 5", gives the mastery line (mastery is 5 of 5) and notes that m rose 6 times while airflow fell by only 56%. Reset then returns to wide open for unscored stepping.

All values are illustrative, chosen to make the pattern easy to see. They are not Twin City Fan published ratings; real fan curves are read from the catalog.

How to Use

  1. Read the wide-open operating point: 10,000 CFM at 5.00 in. wg.
  2. Press Close damper one step. Choose Higher, Lower or Same for airflow and for static pressure, then press Check.
  3. Read the feedback and compare the new operating point (diamond) with the previous one (open circle) on the chart and in the table.
  4. Repeat through Step 5. Then read the summary, press Reset and step through the settings freely; nothing is scored in this explore mode. Start over clears the score and restarts the challenge at any time.

Keyboard: keys 1, 2 and 3 choose airflow higher, lower or same; keys 4, 5 and 6 choose static pressure higher, lower or same; Enter closes the damper one step or checks the prediction. Tab and Space or Enter work on every button.

Data label: "Values are illustrative. Real fan curves are read from the catalog.", shown at the bottom left of the chart.

Iframe Embed Code

You can add this MicroSim to any web page by adding this to your HTML:

<iframe src="https://moroshek.github.io/FanTASTIC-textbook/sims/ch19-damper-throttling-explorer/main.html"
        height="908px"
        width="100%"
        scrolling="no"></iframe>

Lesson Plan

Learning Objective

The learner will interpret how closing an outlet damper moves a fan's operating point along its fan curve, predicting the direction of change in airflow and static pressure before each new damper setting is revealed (Bloom: Understand, verb: interpret).

Audience

Adult professional development: new application engineers, sales staff, representatives and facilities staff.

Duration

10-15 minutes

Prerequisites

  • Fan curve, system curve and operating point (Chapter 10)
  • Damper throttling and the resistance multiplier m (section "Damper Throttling" of Chapter 19)

Activities

  1. Predictions (6 min): Work through the five closing steps. Before pressing Check, say out loud why the new intersection must sit where you predict.
  2. Review (4 min): Read the table from Wide open to Step 5. Compare the change in m with the change in airflow, and notice that pressure rises by less than 1 in. wg in total.
  3. Explore (3 min): Press Reset and step through again, watching only the fan curve. Ask: did the fan curve ever move?

Common Misconceptions Addressed

  • Closing a damper lowers the pressure the fan produces: every step reveals a higher static pressure, and the wrong-pressure feedback explains why.
  • Closing a damper changes the fan curve: the fan curve is drawn once and never moves; only the system curve steepens.
  • Airflow falls in proportion to how far the damper is closed: the summary contrasts the 6-fold rise in m with the 56% drop in airflow.

Assessment

  • Running score "Predictions correct: n of 5"; mastery is 5 of 5 steps with both directions correct, one attempt per step.
  • Viewing the chart after a reveal is exploration, not evidence.
  • Follow-up question: why does damper throttling save less horsepower than other flow-control methods? (The fan still develops more pressure at the throttled point.)

References

  1. Chapter 19: Damper Details and Duct Connections - the "Damper Throttling" section this MicroSim exercises.
  2. Chapter 10: Fan Performance, Curves and Ratings - fan curves, system curves and the operating point.
  3. FanPedia: The Fan & Blower Encyclopedia (PDF) - pp. 182-183 describe outlet damper control shifting the operating point to the left of the rating point, with smaller horsepower savings than other methods.
  4. FanPedia online - Twin City Fan's online fan and blower encyclopedia.
  5. Plotly JavaScript Open Source Graphing Library - the charting library used to draw the curves.